This invention relates to a heating system having means for storing heat. The system has particular and preferred utility in a heating system using solar energy as a source of heat.
Several factors related to the production of heat for structural space heating systems, hot water heatins systems and other heating systems have recently coalesced to encourage new technologies for producing heat in such heating systems. Specifically, the cost of heat energy has continued to rise. In addition, the vast consumption of heating energy, particularly in urban areas, has provided substantial problems of pollution, and, most recently, a shortage of traditional hydrocarbon fuels has developed. Each of these factors has contributed to recently increased interest in sources of heat energy alternative to those traditionally employed and more efficient utilization of heat energy produced from all sources. One such alternative energy source is energy radiated from the sun.
Solar energy has, of course, long been known but has not been significantly exploited for two principal reasons. The first reason is the relatively low density of the energy per unit area of a surface collecting the energy. The low energy density requires both substantial surface areas and relatively long times for the collection of a required quantity of heat energy. The second reason is the uncontrollable availability of energy from the sun to times which may not coincide with the desired times of energy utilization. Specifically, solar energy is only available during daylight hours and even during daylight hours varies in available intensity with the angle of incidence of the energy which, of course, depends upon the time of day, and varies with the degree of cloud cover.
Both of these problems may be mitigated by means for the storage of heat energy accumulated during times of excess availability for later distribution. Such heat storage means may also have utility with heating systems employing traditional energy sources by permitting consumption of the energy during convenient times such as off-peak hours in the availability of electric energy or hours of relatively low pollution.
One system for the storage of heat energy, particularly heat energy from a solar collector, is disclosed in U.S. Pat. No. 3,369,541 issued Feb. 20, 1968 in the name of Thomason. This patent discloses two embodiments of a heat storage device each having a tank containing a heat-storing, fluid medium. A pump circulates the fluid from the tank to a solar-energy heat collector and returns the heated fluid from the collector to the tank for storage. Heat exchange passages adjacent the tank receive a flow of air which is heated in the passages and discharged into a structure as space heat. In only one embodiment a collection of stones or other heat storage and heat exchanger material surrounds the tank in thermal communication with the tank for the storage of heat brought to the tank by the fluid and in heat exchanging relation with the air to be heated by pumping the air through interstices between the stones or other material.
In both embodiments disclosed in the patent the tank for storing the heated fluid medium is internally entirely open to permit uninhibited mixing of portions of the fluid heated to different temperatures, for example, fluid heated at times of different solar energy intensity. Moreover, inlet and outlet passageways for conveying the fluid to and from the solar heat collector are disposed in diagonally opposite corners of the tank; this arrangement would appear to promote a generally rotary, mixing circulation of the fluid in the tank as caused by the jet action of the fluid withdrawn from the tank for heating in the collector and returned to the tank for storage. Mixing differently heated portions of the fluid will degrade the higher temperature of fluid portion heated to the temperature toward the lower temperature of other fluid portions of the tank.
In the patent, water is suggested as the fluid. In the embodiment having stones for the storage of heat, it is believed that the stones, although 21/2 times heavier than water, have a specific heat of only 1/4, to provide a thermal heat storage efficiency only 60% of that of a system utilizing only water for the storage of heat.
Systems for heating hot water with solar energy have been commercially available for a number of years. However, it is believed that these systems have only a heat collector and a tank for the storage of water heated in the collector. A discharge pipe then distributes the heated water to hot water outlets as well known in plumbing systems distributing hot water from water heaters using more conventional energy sources. As with conventional water heaters, it is believed that the tanks storing solar heated water rely on convection currents of the water, with or without an internal pipe structure for directing the convection flow of the water, to maintain a uniform temperature of heated water in the tank. The uniform temperature of the water in such water heaters is considered desirable to provide the maximum quantity of water heated to a desired temperature, usually a temperature preset with a thermostat connected to the water heater.
The relatively low energy density and variable availability of solar energy additionally present another problem. Specifically, a medium heated by a solar heat collector is often heated to a temperature only slightly above that at which it was introduced into the solar heat collector. It is then quite possible that for a large part of a day the solar heat collector could heat the medium to a temperature warmer that that at which it was supplied to the collector but cooler than the warmest temperature to which a portion of the medium was earlier heated. Operation of a system under such conditions will degrade the maximum temperature of the heat storing medium even though additional heat was supplied to the entire system during the heat collecting operation.
This problem is not ordinarily encountered in heating systems utilizing traditional sources of heat energy because these sources of heat energy are selected to provide heating temperatures which are substantially in excess of those required in the system. Moreover, the traditional sources of heat energy usually provide substantially constant heating temperatures. For example natural gas flames at a substantially constant temperature of about 3800.degree. F., a constant temperature substantially in excess of that required from systems for heating structures or hot water.
It is also well-known that the thermal efficiency of heat exchange devices both for the collection and utilization of heat is greatest with the greatest disparity of temperatures between the media between which heat is to be exchanged. It is therefore desirable in a system having means for heating a medium to introduce the medium into the heating means at the lowest possible temperature. Similarly, it is desirable to introduce a medium into means for utilizing the heat of the medium at the highest possible temperature.